fix: make sure header must be an object schema

[derberg]

Description

• According to specification message headers must be defined as a schema of an object. Other types are not allowed.
• Solution based on https://json-schema.org/understanding-json-schema/reference/combining.html#id5

now validation fails with:

"headers": {
    "type": "integer",
    "enum": [1, 2]
}

but it is allowing:

"headers": {
    "type": "object",
    "properties": {
      "my-app-header": {
        "type": "integer",
        "minimum": 0,
        "maximum": 100
      }
    }
  }

Related issue(s)
Fixes asyncapi/parser-js#68

[asyncapi-bot]

🎉 This PR is included in version 2.7.2 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀